As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE APU WAS RUNNING AND THE AIRCRAFT WAS USING THE APU GEN FOR POWER. THE GROUND CREW NOTICED BLUE SMOKE AND A PLASTIC BURNING SMELL COMING FROM THE APU THE GENERATOR HAD GONE OFF LINE. SHUT THE APU DOWN. PLACED THE APU ON MEL 49-00-1. REPLACED APU STARTER/GEN IAW AMM TASK 24-33-01-000/400-801. OPS CKS GOOD NO SMELL OR BLUE SMOKE NOTED. CLEAR DMI. RETURNED TO SERVICE.